# std::basic_const_iterator

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Defined in header `<iterator>` |  |
template< std::input_iterator Iter > class basic_const_iterator; |  | (since C++23)

`std::basic_const_iterator` is an iterator adaptor which behaves exactly like the underlying iterator (which must be at least an LegacyInputIterator or model `input_iterator`), except that dereferencing converts the value returned by the underlying iterator as immutable. Specializations of `std::basic_const_iterator` are constant iterators, that is, the iterator can never be used as an output iterator because modifying elements is not allowed.

### Member types

Member type | Definition
`iterator_category` (conditionally present) | If `Iter` models `forward_iterator`: member `iterator_category` is the same type as std::iterator_traits<Iter>::iterator_category. Otherwise, there is no member `iterator_category`.
`iterator_concept` | std::contiguous_iterator_tag, if `Iter` models `contiguous_iterator`; std::random_access_iterator_tag, if `Iter` models `random_access_iterator`; std::bidirectional_iterator_tag, if `Iter` models `bidirectional_iterator`; std::forward_iterator_tag, if `Iter` models `forward_iterator`; std::input_iterator_tag otherwise.
`value_type` | std::iter_value_t<Iter>
`difference_type` | std::iter_difference_t<Iter>
`reference` (private) | std::iter_const_reference_t<Iter> (exposition-only member type*)

### Member objects

Member name | Definition
`current` (private) | the underlying iterator from which `base()` copies or moves (exposition-only member object*)

### Member functions

(constructor) | constructs a new iterator adaptor (public member function)
base | accesses the underlying iterator (public member function)
operator*operator-> | accesses the pointed-to element (public member function)
operator[] | accesses an element by index (public member function)
operator++operator++(int)operator+=operator--operator--(int)operator-= | advances or decrements the iterator (public member function)
operator constant-iterator | converts into any constant iterator to which an underlying iterator can be convertible (public member function)
operator==operator<operator>operator<=operator>=operator<=> | compares the underlying iterators (public member function)

### Non-member functions

operator<operator>operator<=operator>=operator<=> (C++23) | compares `basic_const_iterator` with non-`basic_const_iterator` (function template)
operator+operator- (C++23) | advances or decrements the iterator (function template)
operator- (C++23) | computes the distance between two iterator adaptors (function template)
iter_move (C++23) | casts the result of dereferencing the underlying iterator to its associated rvalue reference type (function)

### Helper classes

std::common_type<std::basic_const_iterator> (C++23) | determines the common type of an iterator and an adapted `basic_const_iterator` type (class template specialization)

### Helper alias templates

template< std::input_iterator I > using const_iterator = /* see description */; |  | (since C++23)

If `I` models `constant-iterator` (an exposition-only concept), then const_iterator<I> denotes a type `I`. Otherwise, basic_const_iterator<I>.

template< std::semiregular S > using const_sentinel = /* see description */; |  | (since C++23)

If `S` models `input_iterator`, then const_sentinel<S> denotes a type const_iterator<S>. Otherwise, `S`.

### Helper function templates

template< std::input_iterator T > constexpr const_iterator<T> make_const_iterator( I it ) { return it; } |  | (since C++23)
template< std::semiregular S > constexpr const_sentinel<S> make_const_sentinel( S s ) { return s; } |  | (since C++23)

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_ranges_as_const` | `202207L` | (C++23) | `std::basic_const_iterator`
`202311L` | (C++23) (DR) | `std::basic_const_iterator` should follow its underlying type's convertibility

### Example

Run this code

    #include <cassert>
    #include <iterator>
    #include <vector>

    int main()
    {
        std::vector v{1, 2, 3};
        std::vector<int>::iterator i = v.begin();
        *i = 4;   // OK, v[0] == 4 now
        i[1] = 4; // OK, the same as *(i + 1) = 4;

        auto ci = std::make_const_iterator(i);
        assert(*ci == 4);   // OK, can read the underlying object
        assert(ci[0] == 4); // OK, ditto
        // *ci = 13;        // Error: location is read-only
        // ci[0] = 13;      // Error: ditto
        ci.base()[0] = 42;  // OK, underlying iterator is writable
        assert(*ci == 42);  // OK, underlying location v[0] was modified
    }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2836R1 | C++23 | `basic_const_iterator` doesn't follow its underlying type's convertibility | conversion operator provided

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